The present invention is a continuation of PCT/SE2004/000484, filed Mar. 30, 2004, which claims priority to SE 0301136-8, filed Apr. 15, 2003, both of which are hereby incorporated by reference.
The present invention relates to a system and a method for controlling viscosity of a fluid, in particular hydraulic fluid such as hydraulic oil. The method and the system are especially suitable for application in hydraulic systems of heavy vehicles such as loaders and snow-clearing equipment.
Viscosity is one of the most important properties of a hydraulic fluid, especially in a hydraulic operating system. Viscosity is a value of the fluid's flow resistance, or, in other words, the inertia with which the fluid moves. When the viscosity is low, the fluid is thin and free-flowing. Conversely, when the viscosity is high, the fluid is then thick and sluggish.
High viscosity of a hydraulic fluid results in a hydraulic system which is sluggish and can lead to excessive loading of pumps, for example. On the other hand, a viscosity which is too low results in greater risk of leakage in seals and in development of friction-reducing films of oil.
Hydraulic fluid such as oil becomes thicker and acquires higher viscosity when its temperature drops, and becomes thinner, acquiring a lower viscosity, when its temperature rises. That is to say that changes in temperature can have a dramatic effect on viscosity and, consequently, on the functioning of components in the hydraulic system.
In particular, recent environmentally refined hydraulic fluids have been found to have unfavorable properties at low temperatures. A hydraulic fluid at its optimal temperature usually has, in addition to a suitable viscosity, positive properties such as efficient release of air and a desirably high level of incompressibility.
Changes in the temperature of the hydraulic fluid which affect operation of the hydraulic system can be caused by the surrounding environment, such as weather and wind, and by heat generated internally in the system, for example by a pressure drop in valves.
The invention will be described below in connection with working equipment in the form of a wheel-mounted loader. This is a preferred but in no way limiting application of the invention. The invention can for example also be used on other types of operating equipment, such as frame-steered dumpers and excavators.
Historically, hydraulic systems on conventional loaders have been constructed with a continuous pumping circulation, even when the hydraulics are not in use.
Modern machines are often constructed with load-sensing hydraulic systems and then expediently use pumps with variable displacement. In load-sensing systems, there is no pump circulation if the hydraulics are not in use, which means that there is no continuous heating of the hydraulic fluid. In addition, load-sensing hydraulic systems usually have less power dissipation, that is to say these systems generate less heating on account of less dissipation. This is excellent in a hot climate, since the need for cooling is reduced or disappears. However, for example in the case of snow clearing and snow ploughing of roads, the engine has to operate while the working hydraulics are relatively inactive. For example, in the loading and unloading of lorries too, there may be idle times between vehicles with relatively little use of the operating hydraulics.
This means that the surrounding environment will effectively cool the hydraulic fluid, since there is no or very little self-heating for counteracting cooling by the environment. There is then a great risk of complications arising on account of the hydraulic fluid having an unfavourable, excessively high viscosity caused by too low a temperature of the hydraulic fluid.
It is desirable to eliminate the abovementioned disadvantages.
It is desirable to define a method and a system for controlling the viscosity of a hydraulic fluid.
It is desirable to be able to increase the working temperature of a hydraulic fluid in a load-sensing hydraulic system.
Aspects of the present invention relate to a method and system for controlling the viscosity of hydraulic fluid. The viscosity of a hydraulic fluid is usually very much dependent on temperature. By using existing throttles which generate a pressure drop, the hydraulic fluid can be heated by creating a power drain in these existing throttles through a controlled increase of the pressure through them. By controlled heating, the viscosity of a hydraulic fluid can be regulated. According to the invention, the existing throttles are preferably cavitation-preventing openings in valves.
An aspect of the present invention relates to a method for controlling the viscosity of a hydraulic fluid. The hydraulic fluid is included at least in part in a hydraulic circuit. The hydraulic circuit comprises a pump, a first load, and a valve for coupling the first load to the pump and for uncoupling the first load from the pump.
In the uncoupled position, the valve comprises an existing cavitation-preventing opening to the load. That is to say that when the load is uncoupled from the pump with the aid of the valve, there is then a cavitation preventing opening between the pump and the load. The cavitation-preventing opening has a considerably smaller cross-sectional area than the opening of the valve which couples the load to the pump. Although the term “uncoupled” is used, there is therefore still a small flow connection between the pump and the load when the valve is set in the uncoupled position.
According to an aspect of the invention, the method controls the viscosity of the hydraulic fluid by, when necessary, increasing the temperature of the hydraulic fluid via a number of steps. In a first step, the temperature of the hydraulic fluid is determined. In a second step, the valve status is determined, that is to say in which position the valve is located. In a third step, the pressure of the hydraulic circuit is increased in the case where the valve status is such that the first load is uncoupled and the temperature of the hydraulic fluid is lower than a predetermined temperature. In order to increase the pressure, the pump is controlled so that it delivers an increased flow. This creates a temperature-increasing pressure drop in the cavitation preventing opening in the valve. The pump thus pumps hydraulic fluid through the opening, where a pressure drop occurs, and thus a power drain occurs which results in an increase in the temperature of the hydraulic fluid.
The pressure is expediently increased until a predetermined pressure is obtained in the hydraulic circuit, or until a pressure is obtained which is a function of the difference between the predetermined temperature and the temperature of the hydraulic fluid in such a way that the greater the temperature difference, the higher the pressure, or until a predetermined power drain is obtained as heating of the hydraulic fluid via the pressure drop in the cavitation preventing opening. The predetermined pressure can thus either be measured directly via pressure sensors or calculated on the basis of other measurable parameters.
In certain cases it is expedient that the increased pressure is maintained only for a predetermined length of time. In other cases, it is expedient that the increased pressure is maintained for a length of time until the difference between the predetermined temperature and the temperature of the hydraulic fluid is less than or equal to a predetermined value.
In certain applications, the viscosity control has a lower priority than the first load, which means that the viscosity control according to the method is interrupted when there is a need to obtain a power drain in the first load. In other applications, the step of determining the valve status also comprises a subsidiary step after the determination of the valve status, namely, in the case where the valve status is such that the first load is coupled in and the temperature of the hydraulic fluid is lower than a predetermined temperature, the first load is uncoupled and a new determination of the valve status is made, which means that the viscosity control has a higher priority than the first load.
In certain applications, the method can expediently comprise two further steps after the determination of the valve status. In the case where the valve status is such that the first load is coupled in and the temperature of the hydraulic fluid is lower than a predetermined temperature, a first further step determines whether the power drain of the first load or an acceptable lesser power drain can be taken from the flow which is obtained from the cavitation-reducing opening in combination with a higher pressure in the hydraulic circuit. If it is determined that the power drain of the first load or an acceptable lesser power drain can be taken, a second further step uncouples the first load and the pressure in the hydraulic circuit is increased to the necessary value so that the first load can take its power drain or an acceptable lesser power drain in combination with the pressure drop creating heating of the hydraulic fluid.
In certain applications, the hydraulic circuit comprises a second load which has extra high priority and is of short duration. When the second load is coupled in, the pressure in the hydraulic circuit is increased to a predetermined value and the first load is uncoupled, all of this during a length of time which is of the order of a power of ten shorter than the shortest length of time during which an increased pressure exists in the hydraulic circuit for viscosity control.
The hydraulic circuit is sometimes an auxiliary circuit and the first load is a fan. The pump is preferably a controllable load-sensing pump with variable displacement.
The above-described method steps according to the invention can be combined randomly to give a procedure, as long as there are no conflicts between the method steps.
An aspect of the invention relates to a viscosity-controlling system for controlling the viscosity of a hydraulic fluid. The system comprises a hydraulic circuit having a pump, a first load, and a valve for coupling the first load to the pump and for uncoupling the first load from the pump. The hydraulic fluid is included at least in part in the hydraulic circuit. In the uncoupled position, the valve comprises an existing cavitation-preventing opening to the load. The system additionally comprises a control unit which controls the valve and the pump.
According to an aspect of the invention, the system is designed to control the viscosity of the hydraulic fluid by, when necessary, increasing the temperature of the hydraulic fluid. This is achieved by the control unit being designed to determine the temperature of the hydraulic fluid and the valve status and on this basis the control unit is designed to increase the pressure of the hydraulic circuit by controlling the pump in the case where the valve status is such that the first load is uncoupled and the temperature of the hydraulic fluid is lower than a predetermined temperature. This creates a temperature-increasing pressure drop in the cavitation preventing opening in the valve.
The control unit is expediently designed to control the pump so that the pressure is increased until a predetermined pressure is obtained in the hydraulic circuit, or to control the pump so that the pressure is increased until a pressure is obtained which is a function of the difference between the predetermined temperature and the temperature of the hydraulic fluid in such a way that the greater the temperature difference, the higher the pressure, or to control the pump so that the pressure is increased until a predetermined power drain is obtained as heating of the hydraulic fluid via the pressure drop in the cavitation preventing opening.
In certain embodiments, it may be expedient for the control unit to be designed to control the pump so that the increased pressure is maintained only for a predetermined length of time. In other embodiments, it may be expedient for the control unit to be designed to control the pump in such a way that the increased pressure is maintained for a length of time until the difference between the predetermined temperature and the temperature of the hydraulic fluid is less than or equal to a predetermined value.
It is sometimes expedient for the viscosity control to have a lower priority than the first load, which means that the control unit is designed to interrupt the viscosity control when there is a need to obtain a power drain in the first load. It is sometimes expedient for the control unit to be designed, in conjunction with the determination of the valve status, and in the case where the valve status is such that the first load is coupled in and the temperature of the hydraulic fluid is lower than a predetermined temperature, to uncouple the first load and determine the valve status again, which means that the viscosity control has a higher priority than the first load.
In some embodiments, in the case where the valve status is such that the first load is coupled in and the temperature of the hydraulic fluid is lower than a predetermined temperature, the control unit is designed to determine whether the power drain of the first load or an acceptable lesser power drain can be taken from the flow which is obtained from the cavitation-reducing opening in combination with a higher pressure in the hydraulic circuit, and, if it is determined that the power drain of the first load or an acceptable lesser power drain can be taken, it is designed to uncouple the first load and increase the pressure in the hydraulic circuit to the necessary value so that the first load can take its power drain or an acceptable lesser power drain in combination with the pressure drop creating heating of the hydraulic fluid.
The system can be constructed such that the hydraulic circuit comprises a second load which has extra high priority and is of short duration. In this case, the control unit is expediently designed, upon coupling of the second load, to control the pump such that the pressure in the hydraulic circuit increases to a predetermined value and to uncouple the first load. The second load is coupled-in in this way for a length of time which is of the order of a power of ten shorter than the shortest length of time during which the control unit is designed to maintain a pressure increase in the hydraulic circuit for viscosity control.
The system can sometimes be part of a slightly larger hydraulic system and, in this case, the hydraulic circuit can, for example, be an auxiliary circuit. The first load can, for example, be a fan for cooling an engine in a vehicle. The pump can expediently be a controllable load-sensing pump with variable displacement.
The above-described features can be randomly combined to construct a system according to the invention, on condition that there are no conflicts between features.
A great advantage of an aspect of the invention is that an existing installation can be used, as long as there is a cavitation preventing opening. The only thing which is usually needed is a change in the control of the hydraulic system. A temperature sensor is usually present. This means it is incredibly simple to upgrade an already existing system. If it is a system in which the valve with cavitation-preventing opening belongs to a cooling fan for the engine, which cooling fan is controlled by the temperature of the engine's cooling water, then the likelihood of a usage conflict is small. This is because the cooling requirement of the engine in a cold climate/environment is minimal, while at the same time heating of the hydraulic fluid is probably necessary.
The opposite applies in a hot climate/environment where the engine requires a lot of cooling, whereas the hydraulic circuit probably does not require any heating.